Addition reactions photoaddition

Additions to Aromatic Hydrocarbons. A variety of photochemical additions to aromatic hydrocarbons have been reported. Benzene and its derivatives add to maleic anhydride74-76 as well as to simple olefins,77-80 isoprene,81 acetylene derivatives,79,82 and alcohols.83 The mechanism of the maleic anhydride-benzene reaction is discussed in Section IV. A.4. Naphthalene forms a photoadduct with dimethyl acetylenedicarboxylate62 and with acrylonitrile8211 while anthracene behaves similarly with maleic anhydride84 and with 1,2-benzanthracene.85 The photoaddition of several aromatic amines to anthracene has been reported to proceed via a charge transfer complex86,87 in fact, the majority of these addition reactions may proceed in this manner. 

The development of such addition reactions for synthetic purposes remains limited because of the complexity of the photochemical reaction mixtures. Nevertheless, a facile synthesis of dihydroshowdomycin [54] uses the same type of photoaddition of amides on unsaturated sugars following Scheme 26. 

Since it has been observed that the hydrogen atoms attached to nitrogen in amines were not easily abstractable in free radical reactions (6, 74), it may be assumed that the aldehydic part of the formamide molecule will be more reactive in the photoaddition reactions than the amino function, thus leading to the following addition reaction with terminal olefins,... 

The photoaddition of alkanes onto electron-poor alkynes (e.g., propiolate or acetilendicarboxylate esters) can be accomplished by a radical conjugate addition reaction [7]. Radicals have been generated either via hydrogen abstraction from cycloalkanes or via electron transfer from 2-alkyl-2-phenyl-l,3-dioxolanes. In the first case, the irradiation was pursued on an alkane solution of an aromatic ketone (used as the photomediator) and the alkyne. Under these conditions, methyl propiolate was alkylated upon irradiation in the presence of 4-trifluoromethylacetophenone to form acrylate 48 in 97% yield (E/Z= 1.3 1 Scheme 3.31) [78]. 

This review article deals with addition and cycloaddition reactions of organic compounds via photoinduced electron transfer. Various reactive species such as exdplex, triplex, radical ion pair and free radical ions are generated via photoinduced electron transfer reactions. These reactive species have their characteristic reactivities and discrimination among these species provides selective photoreactions. The solvent and salt effects and also the effects of electron transfer sensitizers on photoinduced electron transfer reactions can be applied to the selective generation of the reactive species. Examples and mechanistic features of photoaddition and photocycloaddition reactions that proceed via the following steps are given reactions of radical cations with nucleophiles reactions of radical anions with electrophiles reactions of radical cations and radical anions with neutral radicals radical-radical coupling reactions addition and cycloaddition reactions via triplexes three-component addition reactions. 

Photodimerization of acetylene to give vinylacetylene (butenyne) and formation of polymers in the photolysis of alkynes generally are examples of photoaddition to alkynes. Photopolymerization of di- and poly-ynes has been studied, and for both conjugated diynes or triynes the polymerization process is a 1,4-addition reaction (equation 34). The products are highly unsaturated, and they tend to contain a high proportion of oxygen after exposure to the atmosphere. 

The cycloaddition of the allenes (63) to cyclopentenone results in the formation of the two adducts (64) and (65) in a ratio of 4 1. The addition reaction occurs primarily across the less substituted double bond of the aliens. Cycloaddition of acetylene and but-l-yne to the enones (66) can be brought about using left-circularly polarised light. The resultant cyclobutane derivatives are readily converted to the optically active enones (67). The photoaddition of hex-1-yne to the lactone (68) affords the two isomeric adducts... 

Addition Reactions.- The photoelectron transfer process of the iminium salt (38) with the 3-butenoate anion results in the formation of the allylated product (39). The reaction involves decarboxylation of the 3-butenoate followed by a radical coupling reaction. The photoaddition of halogenated alkenes to the tetraraza phenanthrene (40) yields products (41) of (2+2)-addition. The Eu(III)/Eu(II) photoredox system has been studied with regards to its reactivity toweu ds a-methylstyrene. Irradiation of the system at > 280 nm in methanol yielded the products (42) and(43). ... 

As in the case of photoisomerisations, the reaction can proceed either via the singlet or the triplet intermediate. The component B is part of the reaction in contrast to the sensitisation mechanism discussed above. In addition both pathways via the singlet or triplet state can be quenched. The photophysical steps are summarised in Table 3.2 and the reaction scheme is derived in Appendix 6.6.1.1. In the following examples this information is used to derive the time laws and the quantum yields for 4 types of photoaddition reactions according to either the normal addition reaction... 

The initial report of intramolecular benzene-tertiary amine addition by Bryce-Smith et al. might have been expected to trigger a flurry of research activity similar to that generated by intramolecular benzene-olefin photoaddition reactions. However. a detailed account of the formation of adducts 3 and 4, including isolated yields and complete structural characterization, never appeared. Neither have additional examples of arene-tertiary amine addition reactions been reported. 

These intramolecular addition reactions are remarkable in that they have no intermolecular counterpart. In fact, A/,W-dialky-lamides and tetraalkyl ureas fail to quench styrene fluorescence. However, photoaddition of some 1,1-diarylethylenes and tetra-methylurea has been reported. The intramolecular reactions are proposed to occur via weakly bound nonfluorescent singlet exciplex intermediates, which undergo a-C-H transfer to yield the biradical precursors of the observed products. A triplet mechanism was excluded based on the failure of sensitization by xanthone or quenching by 1,3-pentadiene. The involvement of charge transfer is consistent with the requirement of polar solvents for these reactions. The quantum yields for adduct formation from 19 and 25 are much higher than those of their p-methoxy derivatives, in which the styrene is a much weaker electron acceptor. ... 

This photochemieal reaction occurs through the singlet excited state of 8-MOP. As the singlet state lifetime is short (in the nanosecond range), 8-MOP must be in a close proximity to a pyrimidine base. Therefore, the photoaddition is more likely to involve those 8-MOP moleeules that are intercalated into the double-stranded DNA. This type of photo-addition reaction is supposed to be responsible for die phototoxicity of 8-MOP in human skin (Yang et al, 1989). 

In a few cases alkyl complexes can be synthesized by the photoaddition of dichloromethane or chloroform to a low-valent transition metal complex, but the usual products from these halogen abstraction reactions are the dichloro complexes. Photoinduced oxidative addition reactions are not always the result of the generation of an excited state that is more reactive than the ground state. An alternative photochemical pathway can involve the initial dissociation of a ligand to generate a coordinately unsaturated intermediate, which is then activated toward oxidative addition of the substrate. An example of such a reaction is... 

This chapter contains discussions of photoelimination, photoaddition, and photosubstitution. Although there may appear to be some degree of overlapping between the first two topics in that the species produced by photo-elimination may undergo addition to another substrate, our approach will be to concentrate on the reactions brought about by light absorption rather than subsequent dark reactions. 

Two types of addition to pyrimidine bases appear to exist. The first, the formation of pyrimidine photohydrates, has been the subject of a detailed review.251 Results suggest that two reactive species may be involved in the photohydration of 1,3-dimethyluracil.252 A recent example of this type of addition is to be found in 6-azacytosine (308) which forms a photohydration product (309) analogous to that found in cytosine.253 The second type of addition proceeds via radical intermediates and is illustrated by the addition of propan-2-ol to the trimethylcytosine 310 to give the alcohol 311 and the dihydro derivative 312.254 The same adduct is formed by a di-tert-butyl peroxide-initiated free radical reaction. Numerous other photoreactions involving the formation by hydrogen abstraction of hydroxyalkyl radicals and their subsequent addition to heterocycles have been reported. Systems studied include 3-aminopyrido[4,3-c]us-triazine,255 02,2 -anhydrouri-dine,256 and sym-triazolo[4,3-fe]pyridazine.257 The photoaddition of alcohols to purines is also a well-documented transformation. The stereospecific addition of methanol to the purine 313, for example, is an important step in the synthesis of coformycin.258 These reactions are frequently more... 

Nitramines are known to photodissociate from their jt,jt state to give aminyl and nitric oxide radicals in the presence of an acid the aminyl radicals are protonated to give aminium radicals, which can initiate addition to olefins. As a synthetic reaction, photolysis of nitramines in the presence of acids can be conveniently run under oxygen to give oxidative addition similar to those shown in equation 145 indeed TV-nitrodimethylamine is photolysed with triene 299 under such conditions to give a mixture of 301 and 302, similar to results observed in the oxidative nitrosamine photoaddition169. To simplify the isolation, the crude products are reduced with LAH to form the open-chain amino alcohol 303. Some other oxidative photoadditions of N-nitro dimethylamine to other olefins are reported. As the photoreaction has to use a Corex filter and product yields are no better than those shown by nitrosamines, further investigations were scarcely carried out. 

Primary and secondary amines can add to Cjq as nucleophiles (Section 3.3). Tertiary amines can not form similar addition products, rather an electron transfer imder formation of zwitterions is often observed (Section 3.3). However, a photochemical reaction of tertiary amines with Cjq is possible and leads to complex mixtures of addition products [52-62]. The product distribution strongly depends on conditions such as temperature and the presence of either light or oxygen. If oxygen is thoroughly excluded, 9 is the major product (Figrue 6.8) in the photoaddition of triethylamine [56, 59]. It can be isolated in low yields. 

An example of the equivalent (photoaddition) reaction following hetero-molecular photoassociation is provided by the photochemical addition of maleic anhydride to anthracene." Livingston and coworkers100 have shown that the anthracene triplet state is not involved in this reaction and that, in terms of Eq. (47) in the appropriate form, q%. = 0.03. However, if the excited complex XMQ formed directly by light absorption in the charge-transfer band is the reactive intermediate, this produces the adduct with a computed efficiency of 347 . 

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